The hubs are notably larger than most hubs, but are well finished and look perfectly at home on a high end wheelset.

Want on-the-fly tyre pressure adjustment? Gravaa’s KAPS wheels can do that

These gravel and MTB wheelsets have a pump in the hub to inflate and deflate tyre pressure at the touch of a button.

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One of my highlights so far from this year’s Eurobike is Dutch company Gravaa’s KAPS (Kinetic Air Pressure System) hub-based electronic tyre pressure management system.

Gravaa designed a system to allow a rider to increase or decrease tyre pressure on the move, check their current tyre pressure, and compensate for leakages in the event of a puncture. Tyre pressure is controlled via a Bluetooth button unit mounted on the handlebars and displays the current tyre pressure on either a GPS head unit or smartphone app. 

Gravaa founder Gert Jan van Ginderen set about creating a system capable of inflating and deflating tyres without stopping after watching the lead group of a beach race in the Netherlands disappear up the road when he opted not to stop to adjust his tyre pressure on a tarmac section. 

Four years later, the KAPS system is now available for pre-order, built into Gravaa’s own GX-30 road and XC-28 mountain bike carbon wheelsets. 

The ingenious KAPS system features a miniature, reciprocal membrane pump built into the hub. A camshaft driven solely by the rotation of the wheels moves the pump’s cylinders and a pneumatic clutch activates and deactivates the pump. The clutch is actuated via a pneumatic membrane and deactivated by the pressure inside the tyres.

Gravaa says the KAPS system is highly energy-efficient, with just the rotation of the wheels powering the entire pump system, yet, it can still inflate a tyre by 1 bar (14.5 psi) in just one kilometre for gravel tyres. Larger mountain bike tyres inflate at a rate of 0.5 bar (7.3 psi) per kilometre. To achieve this inflation rate, Gravaa has used pneumatically actuated booster valves to create a large opening for air to pass through the miniature system. Deflation is much quicker, dropping 1 bar in just three seconds. 

A USB-rechargeable battery in each hub powers the electronics and control communications. Gravaa claims the whole system adds just 250 grams to its wheelset when compared with a normal hub, although it didn’t state which hub this comparison is made to. In terms of drag, Gravaa claims the system requires just 7 watts to power the system for inflation, and creates zero additional drag when not in use.

Gravaa has designed its own seven-bolt rotors with a larger bolt diameter to accommodate the large hub shell and electronics. While I can’t comment on the performance of these rotors on a proper ride or longer-term, Gravaa does claim its rotors are lighter than Shimano’s XTR rotors.

Bar the hub, a single spoke, and the valve mount, the remainder of the wheelset is similar to any other wheel, meaning compatibility with disc-brake road, gravel, CX and mountain bikes with no alterations to the bike. All this means riders can also swap out the wheelset at any time if they don’t fancy tyre pressure management on a particular ride.  

The system is controlled by two thumb buttons on the handlebars. The button unit connects to the hub via a Bluetooth connection for a wireless setup. A smartphone app or GPS head unit then displays the current pressure. The smartphone app also offers pressure-management controls, displays the preset pressure increments, and also houses all the system settings, but is not required for the system to function on a ride. The button unit can control the system without a smartphone or head unit connection, and Gravaa does plan to develop a Di2 connection in future.

The Gravaa GX-30 is a carbon, tubeless-ready, 30 mm-deep rim with a 25 mm internal width, weighing in at 1,970 g, including the complete system. 

The XC-28 is Gravaa’s mountain bike offering. Again the XC-28 features a carbon tubeless-ready rim, but shrinks the rim depth slightly to 28 mm, with the internal width increased to 30 mm for the rear wheel. The XC-28 weighs in at 1,990 g. 

The GX-30 is available in axle widths of 100 x 12 mm, 100 x 15 mm, or 110 x 15 mm (Boost) for the front, and 142 x 12 mm or 148 x 12 mm (Boost) for the rear. The XC-28s are available with the 110 x 15 mm and 148 x 12 mm Boost setup. Gravaa is offering Campagnolo, Shimano, and SRAM freehub bodies. Both wheels are compatible with Gravaa’s 140 mm, 160 mm, and 180 mm disc rotors.

Pre-order for both wheelsets is open now at Gravaa.com, with shipping expected to commence in October. Both wheelsets are priced at €2,959, but Gravaa is offering an early-bird 25%-off offer. 

I got the opportunity to very briefly try out the KAPS wheels at Eurobike. The entire system is impressively neat and functional. Sitting in the hot German sun, the system regularly kicked in to maintain the selected tyre pressure before I even set off on the ride.

The smartphone app allows the user to pre-set the pressure bands in 0.1 bar (1.45 psi) increments, with the button unit providing the on-the-move inflation and deflation to these set pressures. 

Once out for a ride, the 1 bar/km inflation rate had me questioning whether the system was working at first, but the pressure reading for each tyre on the smartphone app changes to white when the system is in operation, with green indicating the system is off. The system can inflate the tyres at any speed above 7 km/h (4.3 mph), and while a full kilometre is required to achieve any inflation, obviously, the faster the bike is moving, the quicker that kilometre is covered. The system can deflate the tyre without moving. 

I could hear a noticeable and rather cool gush of air as the system reduced the pressure in the tyre, but the inflation process is either silent or quiet enough that I couldn’t hear it. 

All in all the Gravaa KAPS pressure management system is quite impressive. Would I buy a wheelset for myself? I’m not sure, but it certainly has its benefits for gravel and beach racers, where pressure requirements can vary significantly on changing surfaces. While it undoubtedly also offers benefits for mountain bikers, the system is perhaps not quick enough yet for the rapid pressure changes – even for deflation – that these riders might benefit most from with fast-approaching surface changes. 

Tyre manufacturers and testers could perhaps benefit the most from the on-the-move pressure management. Gravaa claims the KAPS system is accurate to within 50 millibar (0.7psi) – much more accurate than almost any pump. This accuracy, paired with the ability to adjust pressure on the move, could offer much more controlled, reliable, and faster testing. 

For more information, head to Gravaa.com.

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